[Qemu-devel] A second bug in the IO throttling code

[Chris Webb <chris@arachsys.com>]

Whilst you have patches in progress for the queue draining issue with the IO
throttling code which triggers the assert()s in the ide driver, I thought I
should report a second bug I've seen. I'm not sure whether it's related, but
none of the patch series posted so far appear to fix or affect it.

I find that if I start a guest booting linux using extlinux and set a
bytes-per-second throttle value less than about 4MB/s, qemu tends to lock up
completely while the bootloader is loading the kernel. For example, there's
a tiny 10MB ext4 filesystem gzipped up at

  http://cdw.me.uk/tmp/test.img.gz

which just contains extlinux and a kernel. If you run a VM with qemu HEAD as

  qemu -m 1024 -vnc :1 -drive if=none,id=ide.0.0,format=raw,cache=none,file=test.img,bps=10000000 -device ide-drive,bus=ide.0,unit=0,bootindex=1,drive=ide.0.0 -monitor stdio

and watch on VNC, you'll see it hangs whilst loading the kernel. Once this
has happened, no further interaction with the monitor is possible, and the
VNC socket becomes completely unresponsive. This happens about half of the
time with bps set as high as 2*1024*1024.

I first saw this with the version of the block throttling patches I'd
back-ported on top of qemu-kvm 1.0, but have checked that the problem is
still present in HEAD as of this afternoon [361dea401f52].

Best wishes,

Chris.